1. Field of the Invention
The present invention relates to a sprocket provided with a plurality of teeth to be meshed with a chain, and having bottom land portions between the plurality of teeth, each bottom land portion being centered on a tooth bottom and constituted by part of a cylindrical surface of a predetermined curvature radius and a predetermined angular range, and also relates to a chain transmission mechanism having such a sprocket.
2. Description of the Related Art
ISO 606: 1994 (E) stipulates a sprocket tooth profile (referred to hereinbelow as ISO tooth profile) as the conventional standard sprocket of a chain transmission mechanism.
In the ISO tooth profile, as depicted in FIG. 6, where a chain pitch is denoted by p, a pitch diameter is denoted by Dpφ, a roller (bush in the case of a bush chain; same hereinbelow) diameter is denoted by φr, a bottom land portion arc radius is denoted by Rb, a tooth surface radius is denoted by Rs, a root circle diameter is denoted by Dbφ, and the number of teeth in the sprocket is denoted by z, the relationships therebetween are defined by the following equations.Dpφ=p/sin(180°/z)Dbφ=Dpφ−φr Rb(min)=0.505φr Rb(max)=0.505φr+0.069(φr)1/3 Rs(min)=0.008φr(z2+180)Rs(max)=0.12φr(z+2)
As follows from the equations above, in the ISO tooth profile, a bottom land portion 511 of teeth 510 is formed by an arc of a bottom land portion arc radius Rb which is slightly larger than the radius (φr/2) of the roller.
Further, the root circle diameter Dbφ is formed to be equal to a difference between the pitch diameter Dpφ and the roller diameter φr, and the root circle diameter Dbφ is also formed to be substantially equal to a difference between the pitch diameter Dpφ and a two-fold bottom land portion arc radius Rb.
As depicted in FIG. 7A, the standard sprocket 500 is defined as a sprocket in which each roller 521 sits on the center of the bottom land portion 511 when the roller chain 520 is wound therearound.
Since the rollers 521 successively mesh with the bottom land portions 511 of the sprocket 500, the roller chain 520 performs the polygonal movement, vibrates by moving (pulsating) in the vertical direction, and generates noise. Further, in the polygonal movement, the speed in the advance direction of the roller chain 520 also changed.
In order to reduce such vibrations, noise and changes in speed, chain transmission mechanisms using a sprocket with specifications different from those of the above-described standard sprocket 500 have been suggested (see, for example, Japanese Patent Application Publications Nos. 2005-30501, 2005-249166, 2006-170362, 2007-107617, 2008-164045, and 2009-275788).